1. Field of the Invention
The invention relates to sensing the effects of a physical disturbance along a signal path, especially human activity at a fence, buried sensing line or other extended sensing path.
A disturbance produces vibration, impact, acoustic noises, stress and/or pressure variations and the like, locally changing one or more signal paths in a manner that produces a time change in the phase relationships between carrier signals propagating along the signal paths, e.g., one or more optical fibers. These phase effects originate at the point of the disturbance and are carried onward as the carrier signals propagate. Advantageous detection of these phase effects in the present invention allows the location of the disturbance to be discerned.
According to the invention, at least two interferometers are configured and comprise, in part, the one or more signal paths affected by the disturbance. The interferometers produce at least two phase variables in which the phase effects of the disturbance are manifested. The at least two interferometers can comprise the same and/or different interferometer configurations, including, but not limited to Mach-Zehnder, Sagnac, and/or Michelson interferometer configurations. In certain embodiments, the produced phase variables are not directly useful, but they are combined by relationships disclosed herein to produce new composite variables. The relationship between the composite variables enables the location of the disturbance to be discerned. In certain embodiments, this relationship is the time lag between the variations over time of two composite variables that have identical waveshapes over time. The time lag identifies the location of the disturbance in view of the specific layout of the interferometers used. In other embodiments, the ratio of the composite variables identifies the location.
2. Description of the Related Art
Intrusion detection advantageously involves detection of the location of a disturbance that impinges on a boundary such as the perimeter of a protected area, e.g., a person climbing a fence into or out of a secured premises. Aside from sensing a breach of security, it may be desirable to detect activity near a given sensing boundary, or crossing a boundary, or proceeding along a path or other sensing line. Such activities are generally exemplified herein with reference to intrusion detection. Detecting the location of the disturbance refers to determining a point along an elongated line or boundary near or at which activity occurs. The line or boundary is elongated but it might or might not be a straight line. Activity causes a localized physical disturbance, such as vibration, sound waves, stress from the weight of persons or vehicles, etc. It is desirable to detect disturbances quickly and accurately and to identify where exactly the disturbance occurred. With knowledge of the geometry of the elongated sensing path, and the linear point along the path where a disturbance occurs, the location of the disturbance is determined.
U.S. Pat. No. 7,139,476 and parent patent application Ser. No. 11/570,481, filed Dec. 12, 2006 (the US national phase of PCT/US05/11045) concern using the timing parameters of signals affected by a physical disturbance, to calculate the location of a disturbance. The disclosures of said patent and application are hereby incorporated in their entireties. Generally in a device of this description (compare FIG. 1), one or more signals are inserted via couplers or junctions that split and/or combine the signals to produce signal components that are carried in fiber optic waveguides placed to define a detection zone. The fiber optic waveguides might be kilometers long and might be placed along any path, e.g., a straight line or a closed path around an area, or defining a complex array like a raster, or perhaps a three dimensional route through a volume or traversing successive tiers or layers. In the example shown in FIG. 1, solid and dashed lines distinguish the signals that are inserted at either end of a bidirectional path and propagate in opposite directions. An object is to discern the location of a disturbance from the effects of the disturbance on the signal components.
The physical disturbance occurs in the detection zone at some distance L1 from the input end of the first interferometer and a distance L2 from that of the second interferometer. The total distance L1+L2 is a constant, namely the total length. The physical disturbance (e.g., a vibration, a noise, an impact or other physical stress on the fiber optic cable) has a localized physical effect on the fiber optic waveguide. The disturbance modulates the phase of the signal(s) carried in the waveguides. The modulation that is important is a substantially localized time-varying phase shift, typically at a frequency in the range of audible acoustic signals or perhaps including low frequency or higher frequency inaudible signals. The amplitude of the phase modulation typically exceeds the period of the carrier optical signal.
The signals propagating in the same direction have a given phase relationship and the effect of the disturbance is to vary the phase relationship over time, i.e., to produce a shift in the phase relationship between two respective signals. For each pair of signals in FIG. 1, the induced phase variations are designated as φ1(t) for one signal path, and φ2(t) for the other. The relative phase difference or displacement between the two signal paths in the first interferometer (propagating from left to right), detected at time t, will be Φ1(t)=φ(t−t2)+φ01; while the one for the second interferometer (with signal propagating from right to left) will be Φ2(t)=φ(t−t1)+φ02. Here φ(t)≡φ2(t)−φ1(t), t1=L1/c, t2=L2/c, and c is the speed of carrier signal propagation. Furthermore, φ01 and φ02 are defined as the respective contributions of the remainder of the structure to the total phase difference in each interferometer. These contributions φ01 and φ02 typically vary slowly compared to the time scale of variations from a typical physical disturbance (e.g., physical stress due to movement of a person or vehicle), and generally may be regarded as substantially constant.
In previous patent U.S. Pat. No. 7,139,476 and parent application Ser. No. 11/570,481, the measured phase differences Φ1(t) and Φ2(t) are substantially identical waveforms (because they were induced by the same local disturbance on counter-propagating signals in the same signal paths) except for the substantially constant offset φ01-φ02 and a time lag t2−t1 due to the difference in propagation distances from the disturbance, between the two signal directions. The time lag is uniquely determined by the position of the disturbance (and may be zero if L1=L2). By extracting the time lag, for example, by finding a peak cross-correlation between the waveforms Φ1(t) and Φ2(t) at some value of time lag, the position of the disturbance can be measured. This approach will work, provided that the phase responses from the different interferometers have the same waveform shape but are time-shifted.
In FIG. 1, each opposite direction forms an interferometer. The two oppositely oriented signal interferometers in FIG. 1 are each structured as Mach-Zehnder interferometers. In this dual Mach-Zehnder configuration, in each counter propagating direction, a source signal is split by a coupler at one end into components that propagate along two signal legs and interfere with one another at a coupler at the opposite end. The interference signals from the two opposite interferometers do not generally produce intensity waveforms that have the same shape over time.
The Mach-Zehnder interferometer structure shown in FIG. 1, and also other interferometer structures, are known in the art and have been proposed as sensing means, including in fiber-optic-based embodiments, and including in the context of intrusion detection and location. Detectors have been proposed wherein the interferometers are of the same type and also wherein different interferometer types are used. Furthermore, applications of certain coextensive paired or oppositely-oriented overlaid interferometer structures have been proposed for intrusion detection and location, for example, as in Udd, U.S. Pat. No. 5,694,114.
These disclosures in the prior art use the intensities of interference signals as the variables that are measured. However, the time varying shapes of intensities of interference signals in paired interferometer structures are generally different. The intensity signals generally lack a time lag aspect that is uniquely related to the location of the disturbance. The shapes of the intensities can be made substantially the same, if certain conditions are maintained or techniques are invoked, as described in commonly-owned previous U.S. Pat. No. 7,139,476, or the time lag variable can be resolved using phase response signals instead, as described above and disclosed in detail in U.S. Pat. No. 7,139,476 and U.S. patent application Ser. No. 11/570,481.
A technique for inferring the location of a disturbance based on the intensity of interference signals is disclosed in Udd, U.S. Pat. No. 5,694,114, including employing oppositely oriented and overlaid Sagnac interferometers. However, intensity-based techniques such as that of Udd are limited in effectiveness and practicality. For example, in Udd, it is recognized that the technique can only respond to small disturbances. If a disturbance produces phase modulation that is large in amplitude compared to the period of the carrier signal, the proposed intensity-based techniques fail. In practical situations, there is no routine way to limit the magnitude of the disturbance. In fact, in fiber-optic interferometers (such as those described in U.S. Pat. No. 7,139,476 and Ser. No. 11/570,481), the present inventors have discovered that the extent of phase modulation in the detected signals can easily exceed the applicability limit of Udd's small disturbance technique.
Another example was discussed by Stephaus J. Spammer (“Merged Sagnac-Michelson Interferometer for Distributed Disturbance Detection”, Stephaus J. Spammer, Pieter L. Swart, Journal of Lightwave Technology, Vol. 15, No. 6, June 1997), wherein an approach similar to Udd uses the combination of a Sagnac interferometer and a Michelson interferometer. As described above with respect to Udd, Spammer's approach depends on intensity response and is subject to similar limitations.